Development of a light-emitting device using a light-emitting element which has a layer containing an organic material between a pair of electrodes and emits light by applying current between the pair of electrodes has been carried out. Such a light emitting device is advantageous to be thin and lightweight as compared with other display devices that are called a thin display device, and has good visibility since it is self light-emitting device, and accordingly response speed is fast. Therefore, the development of the light-emitting device is actively carried out as a next generation display device, and part thereof is currently put to practical use.
In such a light-emitting element, holes are injected to a layer containing an organic material from an electrode functioning as an anode, and electrons are injected to the layer containing an organic material from an electrode functioning as a cathode. Then, the holes and the electrons are recombined to excite a light-emitting material in the layer containing an organic material, and excess energy is emitted as light when the light-emitting material returns to a ground state. Therefore, the quantity of light emitted by the light-emitting element is proportional to current which flows in the light-emitting element.
Such a light-emitting device which is advantageous to be thin and lightweight is especially suitable for application to a mobile device. A light-emitting device, mounted on a mobile device in which there is a limitation of battery, is required to save power. In addition, a requirement of low energy consumption has been more and more increased in association with environment problems, energy problems, and the like.
There are various methods to reduce power consumption. For example, there are methods of improving internal light emission efficiency, improving external extraction efficiency, reducing driving voltage, or the like. For example, Reference 1 discloses increase of internal light emission efficiency by roughly equalizing injection balance of holes and electrons to reduce power consumption (Reference 1: Japanese Patent Laid-Open No. 2005-116203).